1. Field of the Invention
This invention relates to a linear actuator for linearly driving an output shaft with electromagnetic force and more particularly to a linear actuator so adapted that the output shaft is enabled to produce a prescribed stroke and stop at a prescribed position stably.
Further, this invention relates to a linear actuator so adapted as to permit accurate detection of the position of the output shaft.
2. Description of the Prior Art
The so-called voice coil type linear actuator has found popular acceptance as an actuator for producing a relatively large stroke such as in open/close control of an exhaust gas recirculation (EGR) valve or an air-conditioning valve in a vehicle.
This actuator comes in two types; one type which is provided with a movable magnet formed integrally with an output shaft and a coil stationarily disposed within the magnetic field of the magnet and the other type which is conversely provided with a stationary magnet and an output shaft formed integrally with a movable coil. The actuator of either type is intended to impart a linear motion to the output shaft by producing a relative motion between the coil and the magnet by virtue of the electromagnetic force which is generated by the interaction between the electric current flowing through the coil and the magnetic field of the magnet.
The output shaft is stopped at the position where the resilient force of a spring disposed so as to exert an action in the direction opposite to the motion of the output shaft and the electromagnetic force are balanced. The stroke of the actuator, therefore, is determined by the magnitude of the electric current flowing through the coil and the resilient force of the spring. The stroke is controlled more often than not by adjusting the magnitude of the electric current flowing through the coil.
What is disclosed in the specification of Japanese Utility Model application Disclosure SHO No. 61-117,588 may be cited as one example of the linear actuators of this class.
An actuator having an output shaft connected with a movable magnet has a cross section illustrated in FIG. 11. The output shaft 1 is inserted into the central part of a cuplike magnet retaining member 2 made of such an electrically insulating material such as resin and joined integrally with the retaining member 2 by the use of a nut 3. To the outer surface of the retaining member 2, a magnet 4 is fastened with an adhesive agent.
A movable part 16 composed of the output shaft 1, the cuplike member 2, and the magnet 4 is adapted to be reciprocated as guided by a bearing 6 inserted in a casing 5 and the outer surface of a cylindrical member 7.
One end of a coil spring 8 engages with a screw member 9 which is put inside the cylindrical member 7 so as to adjust the amount of strain to be imparted in advance to the spring 8, while the other end thereof engages with the bottom part of the magnet retaining member 2. As the result, the resilient force of the coil spring 8 urges the movable part 16 including the retaining member 2 in the direction of the bearing 6. The pressing force or the resilient force generated by the coil spring 8 can be adjusted by moving the screw member 9 forwardly or backwardly in the direction of extension/contraction of the spring 8 or the axial direction of the output shaft 1.
A coil 12 is disposed in a gap between the casing 5 and the cylindrical member 7. The coil 12 is set in place in the vertical direction by being nipped with retaining plates 10a and 10b or by molding.
One end of the coil 12 is connected to a lead terminal 14 and the other end thereof (not shown) is connected to the other lead terminal. The lead terminal 14 is fixed in a terminal fitting plate 15 made of an electrically insulating material.
In the actuator constructed as described above, when an electric current is supplied to the coil 12, the electromagnetic force generated by the interaction between the electric current and the magnetic flux of the magnet 4 exerts an action capable of driving the movable part 16 in the direction of compressing the coil spring 8. The spring is compressed as the movable part 16 is moved. The movable part 16 stops at the position where the electromagnetic force and the resilient force of the spring 8 are balanced.
The stroke of the actuator or the stop position of the output shaft 1, therefore, is determined by the magnitude of electric current of the coil 12 and the resilient force of the spring.
The conventional linear actuator described above has entailed the following disadvantage.
The practice of producing a prescribed electromagnetic force by adjusting the electric current flowing through the coil 12 has found general acceptance. In spite of the effort to obtain the prescribed electromagnetic force by the control of the electric current supplied to the coil -2, it is difficult to obtain the prescribed stroke accurately where the resilient force of the spring 8 to be balanced with the electromagnetic force lacks constancy.
It has been customary, therefore, to adjust the resilient force of the spring 8 by moving the screw member 9 in the direction of expansion/contraction of the spring and consequently effecting fine adjustment of the strain (bias) to be preserved in advance in the direction of compression of the spring 8. The problem persists, however, that no fully accurate adjustment of the resilient force is obtained solely by the fine adjustment of the resilient force with the screw member 9.
The resilient force F increases in proportion as the amount of compression x of the spring increases. The relation between the compression x and the resilient force F is expressed by the formula, EQU F=K.multidot.x+b
wherein k stands for the spring modulus and b for the constant depending on the strain given to the spring in advance. While the constant b can be adjusted by the screw member 9, the spring modulus is a constant peculiar to the spring and cannot be adjusted.
When a spring possessing an end face not perpendicular to the center line thereof is compressed, the compression does not produce the designed resilient force in conformity with the spring's inherent spring modulus as mentioned below.
FIG. 12 A is a diagram illustrating the shape of the end face of the spring 8 and FIG. 12 B is a diagram showing the relation between the resilient force of the spring 8 and the force actually acting on the screw member 9.
The spring 8 illustrated in FIG. 12 A is in the state free from the compressive force and has the end face thereof not lying perpendicularly to the center line 8a. The spring modulus of the spring is k. When the spring 8 is compressed, since the compression exerts an uneven compressive force on the end face of the spring 8 as illustrated in FIG. 12 B , the center line 8a of the spring 8 is curved as indicated by the chain line 8b. As the result, the resilient force Fa corresponding to the spring modulus k is resolved into the component Fb directed perpendicularly to the end face of the spring 8 and the component Fc directed parallelly to the end face. On the screw member 9 which is held in contact with the end face of the spring 8, only the component Fb is effectively exerted.
Specifically, only the component Fb of the force Fa determined by the inherent spring modulus k is allowed to act as the resilient force on the screw member 9, depending on the angle which the end face of the spring 8 formed relative to the center line. Since the degree of curving of the center line of the spring increases in proportion as the compression force put on the spring 8 increases, the ratio of the component Fb to the force Fa gradually decreases to bring about an apparent decrease in the spring modulus k.
The resilient force acting on the screw member 9, namely the resilient force exerted in the axial direction of the output shaft 1, is affected by the angle which the end face of the spring 8 forms relative to the center line as described above. A variation, if any, in this angle, therefore, results in impairing the constancy of the resilient force in spite of the accuracy with which the electric current supplied to the coil 12 is effected. This fact renders it difficult for the stroke of the output shaft 1 or the position of stop thereof to be controlled accurately.
The prior art further has the following disadvantages.
(1) For the movable part 16 to be effectively driven by virtue of the electromagnetic force, a suitable gap must be kept between the coil 12 and the magnet 4 and the gap is desired to be uniform throughout the entire circumference of the magnet.
The uniformity of the gap between the coil 12 and the magnet 4 may possibly be impaired when the coil 12 is fixed eccentrically within the casing 5 or a large clearance is provided between the bearing 6 and the output shaft 1 or between the magnet retaining member 2 and the guide member 7. In the absence of uniformity of the gap, the electromagnetic force does not uniformly act on the movable part 16 and the movable part 16 receives an unbalanced driving force.
During the sliding relative motion between the guide member 7 and the magnet retaining member 2, for example, an eccentric load is exerted on specific parts and the relative motion between the magnet retaining member 2 and the guide member 7 does not proceed smoothly, possibly with the adverse result that the responses to the start and stop of the supply of electric current to the coil are delayed. A similar disadvantage arises with respect to the sliding motion which the output shaft 1 produces relative to the bearing 6.
(2) When the stroke is to be controlled by the adjustment of the electric current supplied to the coil, the resistance of the coil is varied under the influence of the heat generated by the current itself in the coil and the ambient temperature and the desired magnitude of electric current and the desired electromagnetic force are not obtained by the application of the prescribed voltage to the coil. This fact makes it difficult for the stroke of the actuator and the position of stop thereof to be stabilized.
(3) The electromagnetic force is possibly varied by the dispersion of resistance of the coil, the variation in the resistance of the coil owing to the heat generated in the coil by the current therein and the influence of the ambient temperature, or by the dispersion of the magnetic field of the magnet and the variation thereof by aging. Since the electromagnetic force to be produced is varied by the variation of the magnitude of resistance of the coil and or the magnetic flux of the magnet, it is difficult to attain accurate control of the stroke of the actuator and the position of stop thereof.